Let the signaure be for example
extern XExtDisplayInfo *XextAddDisplay(
XExtensionInfo* /* extinfo */,
Display* /* dpy */,
_Xconst char* /* ext_name */,
XExtensionHooks* /* hooks */,
int /* nevents */,
XPointer /* data */
);
then you can test
python canlink.py "void* XextAddDisplay(void*, void*, void*, void*, int, void*);" -- -L. -lXext